Comments

You would say "rums" if you're truly using the plural. But that wouldn't happen that often -- "rum" being a mass noun, like "hair" -- "hair" does have the plural "hairs," but how often is it used? I'm drinking some rum, I'll have more rum, come and have some rum with me, we'll drink a lot of rum?

I suppose the time to use "rums" would be if you were referring collectively to more than one kind of rum at once -- like "wines" for referring to more than one kind of wine.

But "deer" is completely different. It's just a noun whose plural form is the same as its singular.

You use "hairs" in sentences like, "The fairy godmother told Prince Florimel he must get three hairs from the tail of the unicorn."

You're absolutely right about "rums." A representative example might be, "I just stocked my liquor cabinet with two vodkas (Stolichnaya and Absolut), two mixers (Bloody Mary and Margarita), two tequilas (one with the worm and one without), and two rums (one white plain and one dark spiced).

My experience with the M-W is that they tend to try to describe the way the language is spoken, rather than to try to prescribe how the language should be spoken. So they straightforwardly accept a lot of things that other dictionaries, as well as many careful speakers, would consider substandard.